British Pat. No. 1,363,075 disclosed that a useful .beta.-lactamase inhibitor could be obtained by the fermentation of certain strains of Streptomyces olivaceus. Until our present invention, it was believed that the material disclosed in British Pat. No. 1,363,075 was substantially pure. However, we have discovered that this is not so and that a very minor component of that material can be isolated and has extremely potent .beta.-lactamase inhibitory activity. This new material is designated MM 4550A and it is believed to be responsible for a major part of any .beta.-lactamase inhibitory activity present in the material of British Pat. No. 1,363,075 although it is responsible for only a very minor part of the antibacterial activity which may be exhibited by that material. Naturally, nothing in this specification should be construed as claiming material as disclosed in British Pat. No. 1,363,075. Other .beta.-lactamase inhibitors are known to be produced by strains of Streptomyces olivaceus, for example those disclosed in German Published Patent Application No. 2340005, but such materials do not have the potent .beta.-lactamase inhibitory activity of substantially pure MM 4550A. U.S. Pat. No. 3950357 disclosed that Streptomyces cattleya produced the antibiotic Thienamycin which has the structure: ##STR1## Based on the present knowledge of the structure of MM 4550A, significant structural differences are apparent between thienamycin and MM 4550A.